For example, it is publicly known to continuously produce propylene oxide through epoxidation reaction by supplying an organic peroxide and propylene to an epoxidation reactor in which a solid catalyst such as titanium-containing silicon oxide is packed (e.g. EP 0345856 A). However, there were problems that a pressure loss of a solid catalyst layer increased with a continuous operation time and the catalyst was destroyed when the pressure loss was over the pressure resistant strength of the catalyst, and that the production amount had to be decreased through suppression of the amount to be supplied of the raw material for avoiding the destroy of the catalyst.